Problem: Mr. Stewart's math class has 16 boys, and 22 of his students bring their calculators to class.  If 13 of the students who brought calculators are girls, then how many boys didn't bring their calculators to class?
Solution: We can solve this using basic arithmetic: If 13 of the students with calculators are girls, and 22 students total have calculators, then $22 - 13 = 9$ of the students with calculators are boys.  So if 9 boys have calculators, and there are 16 boys total, then $16 - 9 = \boxed{7}$ boys don't have calculators.  Alternatively, we could solve this problem using a Venn diagram.